Most electrical appliances have adopted electronic circuits for controlling the appliances as a result of the development of science and technology. Such electronic circuit is usually designed to include a type of electronic circuit board, and the electronic components on the circuit board, such as resistors, capacitors, transistors, and integrated components etc., can be normally operated only under waterproof conditions. Moreover due to the requirement for operation, the electronic circuit for controlling is usually provided at the exterior of the electrical appliance and thus is liable to contact with water and be damaged by the humidity. For example, the control circuit of outdoor Christmas lamps is usually placed in an environment of low temperature and humidity, and such a circuit needs a special design to avoid damage caused by the humidity. Therefore, how to prevent an electronic circuit from being damaged by humidity is an important countermeasure to be considered in designing electronic appliances.